A known image forming apparatus has a calibration function that performs a density measurement of an image to be formed and adjusts the image based on the density of the image. In the density measurement, for example, a density pattern, which includes a plurality of density patches having different densities, is formed. The density pattern is measured by sensors, and density adjustment data is obtained from the results of the measurement. Further, when an image is formed after the density measurement, the density of the image is adjusted to an optimum density by using the density adjustment data.
On the other hand, when an elapsed time or a number of printed sheets from the previous the density measurement exceeds certain criteria, that is, when a predetermined measurement execution condition is satisfied, the density measurement is performed. It is possible to secure image quality by performing the density measurement at an appropriate time.